Another Test: One Tiny Spot of Condensation (Frost)

Thursday it was approximately -1 degree Fahrenheit outside (with bad wind chill). COLD!!!  We didn’t do the whole measuring thing (temps & humidity), but Steve did reach his hand through the wall insulation to feel the siding.

He found one tiny (less than 1 inch square) spot of condensation (frost) on the interior surface of the metal siding. It is not enough to cause damage, but why was it there?

After inspecting closely, Steve realized the insulation where he has been reaching through to test was only 3 – 4 inches thick instead of 6 inches. He had disturbed the loose-fill insulation enough, reaching through with these tests, to decrease the insulation thickness by 30 – 50%. That’s a significant decrease in insulation.

All evidence points to this decrease in insulation as what caused the tiny problem.

I think one could think of this (decreased insulation thickness) as maybe a type of “partial” thermal bridge. It was letting out heat at a much more rapid rate.

It’s also well-known that insufficient insulation can cause condensation problems. I have never read an explanation for why this happens, but I would guess that too much heat escapes, bringing vapor with it. With less heat escaping (when insulation is sufficient), apparently less vapor goes to the cold surface of insulation. Or maybe it’s as simple as less insulation means less air barrier? Either way, all the places where our insulation is good (6 inches of loose-fill sheep’s wool insulation, R value of at least R-19) has been dry.

I used to think that the insulation would keep the interior surface of the siding warm. This has not been shown to be true at all, but rather the vapor is just not (usually) getting to the cold surface of the siding. It again seems to be proving true the (wonderful) truth that vapor diffusion through walls in cold winter is usually not a concern.

I find it interesting that this remains true even with our plaster walls (which are breathable) even with no paint, wallpaper, vapor barrier, or vapor retarder. However, I must point out that it could be our wool insulation helping out. I do NOT know if someone would get the same results with metal siding, plaster walls, & our cold temps if they used another type of insulation. I would think maybe so–that they would be fine also, because I keep reading that vapor diffusion through walls in winter is rarely a problem, but I do not know.

Another odd thing happened that day that has not happened before or since: some condensation appeared on the laundry room window from the shower in the next room. It was very windy that day. Maybe this caused greater air flow from the bathroom? We always open the bathroom window and turn on the bathroom fan after showers. IF the wind blew hard enough through the bathroom’s eastern window, it could blow bathroom air into the laundry room, which sits just west of the bathroom with the (usually open) bathroom/laundry door right in the wind path from the window.

However, I still do not think this condensation problem would have happened without the insulation being messed up. This is because the spot was SO tiny, and everything around it was dry, on the siding, insulation, etc. If the extra humidity were really the problem, I figure the condensation (frost) spot would have been a lot bigger. But apparently the vapor could only get through that one, thinner-insulation spot.

PS The most surprising statement I read recently is that double-stud walls, which are twice as thick, can present more of a risk of condensation on the interior surface of the siding or sheathing. So while good insulation minimizes condensation risk, MORE insulation may not be better. This may (I’m not sure) be due to the greater volume of air within the wall cavity. Others say the greater risk is due to colder sheathing in double stud walls. As our metal siding is already extremely cold, and not experiencing problems, this would not seem to me to be a factor for metal siding, all other things being equal. But, I wouldn’t want to try the double-stud system without knowing about the greater volume of air being a risk factor or not. One could consider interior vapor barrier to help, however, the risks of double vapor barrier (with metal siding) or interior solar vapor drive (with reservoir cladding) are still there. So far in my research & thinking, my educated guess would be that ventilated vinyl siding and/or a rainscreen might help, but I don’t know. I still have questions about rainscreens which no one and no article has answered so far. Though from seeing pictures and reading more articles, it does seem rainscreens work better than I have hypothesized.

PPS Interestingly, with metal siding directly touching our insulation, we can find condensation immediately, versus if we had wood sheathing, in which case the condensation could soak into the wood and be difficult to find.

PPPS I am wondering if it truly IS important that our walls are plaster rather than drywall (with the increased condensation risks of metal siding). This is because I am reading about air entering walls through leakage, in cracks around electrical boxes and between drywall and the wall’s bottom plate. There are no cracks with plaster because plaster fills in spaces, sort of like how loose-fill or spray foam insulation fills in spaces compared to batts. The exception with plaster would be if one is installing a flooring which needs room at the bottom to expand and contract. Ours does not, since it is ceramic tile installed on an insulated cement slab.

Wow, just goes to show once again how many variables there are in wall assembly equations!!!! It seems to even include flooring choices.

Are you tired of the science jargon yet? 🙂  Obviously I do not usually talk this much “science,” but for those of us so sick from mold, answers to these moldy building problems need to be found. It is very concerning to me that mold is found in so many NEW constructed houses. This is not good.

At times I’ve felt my head was going to explode from researching, trying to figure this stuff out, and explaining it! LOL But it’s worth it.

Upcoming post: short, sweet, and about one of my favorite children’s books.  😀

Christa Upton  Black Hills Picture Books   Edgemont, SD  57735